1. Field of the Invention
This invention generally relates to solar cells and, more particularly, to a back contact solar cell using a hybrid organic/inorganic perovskite material.
2. Description of the Related Art
Solar cells based on a combination of organic and inorganic (i.e. organic/inorganic) perovskite materials represent a recent breakthrough in the modern solar cells technology. They have shown a promising power conversion efficiency (PCE) of above 18% in several lab scale cells, using a cost-effective fabrication process and a simple cell structure. In theory, a further improvement of the PCE to 25% is possible, making the perovskite solar cell a desirable technology with a lower cost and higher performance than many other photovoltaic technologies.
FIGS. 1A and 1B are, respectively, mesoporous and planar structure perovskite solar cells (prior art). In general, to achieve appreciable power conversion efficiencies (PCEs) two possible architectures have been adopted for the perovskite materials. The first is a dye-sensitized solar cell (DSC)-style device that comprises: a mesoporous semiconducting metal oxide 106 (e.g., titanium oxide (TiO2)); a perovskite material 108; an organic hole transporting redox material (HIM) 110 to transport positive charges (holes) from the perovskite to the counter electrode; and a gold (Au) or platinum (Pt) counter electrode 112, see FIG. 1A. The planar heterojunction-type device of FIG. 1B has a planar wide band gap n-type semiconductor material 104, such as TiO2, ZnO, etc., on a transparent conductive electrode such as fluorine-doped tin oxide (SnO2:F) (i.e. FTO) or indium tin oxide (ITO), on glass substrate 102, a directly deposited perovskite material 508 on the planar n-type semiconductor 104 as the light absorber layer; an organic HTM 110 on top of the absorber layer 508, and a counter electrode layer 112. CH3NH3Pbl3-XClX is one example of a perovskite.
Overall, the organic/inorganic perovskite material based solar cell combines the technical merits of both the solid-state dye-sensitized solar cell (ssDSC) and the thin film solar cell (TFSC), and represents the trend of solar cell development in recent years. However, the architecture of a perovskite based solar cell is limited by the use of an organic hole transporting material. Other types of cell architectures, such as the Schotty-type, without a hole-transporting material, result in diminished performance. Besides 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (Spiro-OMe-TAD) as the HTM, several other organic alternatives (P3HT:PSS, for instance) have been suggested. The use of Spiro-OMe-TAD as the hole extraction material provides simplicity in deposition, tolerance to the non-smooth interfaces, and most importantly, it is compatible with the perovskite material, as there are no chemical reactions. However, the use of an organic HTM can significantly restrict the application of this technology due to its relatively low long-term stability and sensitivity to moisture. Thus, a stable inorganic HTM with comparable or better properties than Spiro-OMe-TAD is needed. CuSCN has been reported in the literature as the only inorganic HTM used in a sensitized architecture, yielding about 6% PCE.
An alternative to the conventional cell architecture is presented in parent application Ser. No. 14/320,691, using a wide bandgap p-type semiconductor, such as the oxides of molybdenum, vanadium, tungsten or nickel, as the hole extraction layer (functions as an electron blocking layer) to replace the organic HTM. With a proper p-type wide bandgap oxide semiconductor, this cell structure performs similarly to most thin film solar cell structures, such as (CuInx(Ga(1-x)Se2) (i.e. CIGS) or copper zinc tin sulfide (CZTS). Furthermore, inorganic wide bandgap oxide semiconductors provide better cell stability and moisture resistance, as compared to organic HTM cell. However, such a cell structure still requires the deposition of the selected wide bandgap oxide semiconductor over the perovskite. In many cases, the formation of a metal oxide with the proper crystalline structure requires a high temperature treatment, at which the hybrid perovskite materials are not thermally stable. Thus, perovskite layer applies restrictions in the selection of a p-type semiconductor material, as well as formation method.
It would be advantageous if a hybrid organic/inorganic perovskite solar cell could be fabricated with the advantages of back contacts, and without the disadvantages of using an organic HMI material.